1. Field of the Invention
This invention relates to a gas liquefying method, and more particularly a method for liquefying gas containing at least one kind of component of low boiling point, natural gas, for example.
2. Description of the Related Art
As a method for liquefying natural gas, a gazette of Japanese Patent Publication No. Sho 47-29712, for example, discloses a liquefying method in which a methane enriched gas feeding flow is heat exchanged in sequence with a refrigerant of single component under a condition of low temperature so as to be pre-cooled, in turn a condensed part and a vapor part of the refrigerant having multi-components pre-cooled until the part is condensed through a heat exchanging operation with the aforesaid single component refrigerant are separated from each other, in the first stage the aforesaid condensed part is further cooled and expanded, thereafter it is heat exchanged with the aforesaid pre-cooled feeding flow and passed, and in the second stage the aforesaid vapour part is liquefied and expanded, thereafter it is heat exchanged with the aforesaid feeding flow and passed. Referring now to FIG. 5, a main exchanger which acts as its major segment will be described, wherein a heat exchanger 100 has its lower segment acting as the first stage (a high temperature region) 101 and its upper segment acting as the second stage (a low temperature region) 102. After the gas feeding flow is pre-cooled with the single component refrigerant, it is further cooled with the aforesaid single component refrigerant, thereby the pre-cooled gas flow 78 after the condensed component having a high boiling point is removed is fed from the lower part of the flow passage A arranged at the high temperature region 101, in turn, both a high pressure vapour stream (vapour part) 58 and a high pressure condensed liquid flow (a condensed part) 59 in which the multi-component refrigerant partially condensed through a heat exchanging with the single component refrigerant is separated into gas and liquid are also fed into each of the lower segments of the flow passage B and the flow passage C arranged at the high temperature region 101. The high pressure condensed liquid flow 59 of the multi-component refrigerant is further cooled while ascending in the flow passage C in the high temperature region 101, thereafter the liquid passes through an expansion valve 103, is sprayed from a spray nozzle 105 into the high temperature region 101 so as to cool fluids in the flow passages A, B and C. The high pressure vapour flow 58 of the multi-component refrigerant flowing in the flow passage B is cooled there and liquefied, thereafter fed into the flow passage F in the low temperature region 102, and further cooled there and then the flow passes through the expansion valve 104, sprayed from the spray nozzle 106 into the low temperature region 102 so as to cool the fluid in the flow passages E, F. The gas flow 78 flowed in the flow passage A in the high temperature region and cooled therein is fed into the flow passage E in the low temperature region 102, further cooled there, extracted as liquefied gas 60 and recovered as a product. The high pressure condensed liquid flow 59 of the multi-component refrigerant and the high pressure vapour flow 58 of the liquefied multi-component refrigerant sprayed from each of the spray nozzles 105, 106 are completely gasified through a heat exchanging operation with the fluid flowing in the flow passages A, B, C and the flow passages E, F, the gasified multi-component refrigerant vapour flow 68 is compressed by a compressor, thereafter it is heat exchanged with the single component refrigerant at the heat exchanger, circulated and used as the partial condensed multi-component refrigerant (not shown). In this method, a Hampson type heat exchanger is employed as a heat exchanger for the pre-cooled gas feeding flow and the multi-component refrigerant. This Hampson type heat exchanger has a disadvantage that a long flow passage of the heat exchanger is required and a high pressure loss is also resulted due to its manufacturing process in which an aluminum tube is wound around a core pipe in many turns, so that a high compressor horse power for this operation is required and so the heat exchanger by itself becomes large in its size due to the aforesaid structure. In addition, since the low temperature end of the low temperature fluid is present at the top part of the heat exchanger, the refrigerant liquid at the low temperature end is flowed reversely toward the high temperature end by its gravity in the case that the flow of fluid within the heat exchanger is stopped, a heat exchanging operation is carried out between the refrigerant liquid and the high temperature refrigerant vapour accumulated at the bottom part of the heat exchanger so as to cause a rapid boiling of the low temperature liquid to be generated and so it has still a problem in view of its safety.
A gazette of Japanese Patent Publication No. Sho 54-40764 discloses a method for liquefying natural gas in which the refrigerant containing multi-component is not pre-cooled with the single component, but cooled until it is partially condensed through a heat exchanging operation with cooling water, the condensed part and the vapor part of the refrigerant containing pre-cooled multi-components are separated and then the separated condensed part and vapour part are mixed again and fed into an inlet port of the plate-fin type heat exchanger, and further it is flowed in parallel with a flow of cooled component, natural gas, for example, and flowed in opposition to the flow of low temperature refrigerant after the high temperature refrigerant containing mixed condensed part and vapour part is cooled and expanded. Since this method is carried out in such a way that the condensed part and the vapour part of the refrigerant containing multi-components are mixed to each other at the inlet port of the heat exchanger, passed within the heat exchanger as mixed phase and not only the vapour part but also the condensed part are super-cooled down to the temperature in the low temperature region, its heat exchanging amount is increased more and a large-sized heat exchanger is required as compared with that of the method disclosed in the gazette of Japanese Patent Publication No. Sho 47-29712 in which the condensed part is not required to be super-cooled to the temperature of the low temperature region. In addition, since the condensed part contains a large amount of high boiling point components, a temperature difference between a condensing curve for the fluid to be cooled and an evaporating curve for the refrigerant may produce a certain clearance at the high temperature region where the evaporating latent heat of the high boiling point component is utilized to influence efficiently against a design of the heat exchanger, although at the low temperature region where the condensed part is super-cooled, only sensitive heat of the high boiling point component in the refrigerant is utilized, resulting in that it is hard to get a wide clearance at a temperature difference between the condensing curve for the fluid to be cooled and the evaporating curve for the refrigerant and so this process can not be defined as an effective utilization of heat of the refrigerant. Due to this fact, this method has some disadvantages that it requires a higher compressor horse power as compared with that of the aforesaid prior art and an energy consumption is increased.